Does it even matter whether a particular destination that already is served by one of CO's partners is a codeshare? From a mileage-earing potential, the EQM's and RDM's earned flying AF via CDG, for example, would be the same whether one booked the ticket through CO or AF. Same for DL. In fact, when it comes to KL, it might be better to book directly with KL rather than booking the flights as CO codeshares because low KL fare classes earn 100% EQM's.

From a reward redemption perspective, I do not see why it would make any difference whether the route was a codeshare; there still must be reward availability in the passenger's desired class of service and reward type. That is, unless routes that are codeshared with CO somehow have more reward availability, which they do not, to my knowledge.

So, why would it matter whether the new DL flight is a CO codeshare? If you purchase a ticket on the flight, you will earn the same amount of EQM's and RDM's that you would if it were not a codeshare. If you redeem a reward on the flight, it will cost the same as non-codeshared flights to CAI.

The same analysis applies to AF and KL, except that if purchasing a low coach fare on KL, it is better not to purchase the ticket directly from CO in order to fly on a 100% EQM-earning KL fare class.

Am I missing something? What substantial benefits are there to booking already existing flights as codeshares as opposed to non-codeshares? Is it just to be able to book such flights on a single ticket?